I have a question of coordinate system after going through available
documents. Correct me if I am wrong.
Accodring to the CF-Convnetions, parameters such as projection
approaches (eg. Lambert_Conformal_projection) are specified. However, I
find no information about the DATUM. Although it is assumed to be
WGS84, there is possiblity for data in other datum such as Bessel-1841
specified in RT90. My question is, why not make the DATUM as a standard
parameter? With all the effort being made by ncML, the inconsistency
with other correctly geo-referencing dataset is still possible due to
the lack of DATUM parameter. Am i naive or missing/misunderstanding
something on the issue?

One crucial point is that the ncML is the metadata of netCDF files.
Information in ncML is orginated from netCDF. It's true that users can
edit ncML to make it more complete, however, it still depends on the
header of netCDF. Suppose, a user trying to create a data in his
national coordinate system (eg. RT90) using CF-Conventions. The user
specified the projections and relevant parameters. It seems fine at
first. However, when the data is distributed and people who download the
data can't be able to know that the datum is Bessel-1841 which specified
in RT90. The system would transfer the coordinate system using WGS84 as
datum. That, eventually, would lead to inconsistency with other data.

You can add/change metadata using NcML. The Java library can read the NcML
file, which points to the original netcdf file. The C library will eventually
be able to do this also. To distribute the modifications as a netcdf file, you
will have to rewrite the netcdf file, which can be easily done with the Java
library.
John
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